The present invention relates broadly to an interference test apparatus, and in particular an apparatus for locating intermodulation interference sources.
The density of electromagnetic emitters and receptors which are located in the modern military environment has increased rapidly with the development of such large scale military systems as communications, radar, and electronic counter measures. As an example of an environment which would be subjected to such an interference condition, consider the E-4 aircraft. This aircraft utilizes approximately forty-four different systems which are readily identifiable and each of which are potential sources of problems. The transmitter output power levels range from 10 W (+40 dBm) to 1000 W (+60 dBm). The receivers exhibit sensitivity levels from -100 dBm to -135 dBm. Thus, the potential transmitter-to-receiver isolation which would be necessary to avoid interference, ranges from 140 dB to 195 dB.
The interference which is caused by intermodulation (IM) and harmonic products that are generated in nonlinear active devices such as transistors, diodes, and other devices containing semiconductor PN junctions, has been comprehensively researched and examined. However, recent evidence indicates that in operational situations where multiple high power RF signals coexist with sensitive receivers even some passive devices may exhibit sufficiently nonlinear behavior to produce this type of interference. An example of such passive devices in which intermodulation interference is generated are the metal-insulation-metal junctions that occur in the packaging of large scale systems.
The generation of intermodulation products in passive devices arises because most metals in air intrinsically possess a thin layer of oxidation which can and does act as an insulation. When two metallic bodies are joined, a metal-insulator-metal (MIM) interface is produced. Before contact the insulating materials act as a dielectric. Under light pressure the oxidation layer functions as a semiconductor junction. Under increased pressure of contact, the layer is penetrated and the properties of the junction change rapidly. Two programs were conducted at Georgia Tech to characterize the intermodulation generation properties of metal-to-metal interfaces that were found in coaxial cables and connectors, as well as, metal-to-metal interfaces which are similar in characteristics to those found in an aircraft fuselage panel bonds. In particular, it has been found that coaxial connectors were shown to be possible sources of interference products whose levels can be as high as -100 dB relative to the applied signals in the HF to UHF frequency region. Similar tests which were performed on various overlapping metal panels, produced intermodulation product levels that ranged as high as -75 dB relative to the applied signals. Thus, in operational situations which require a transmitter-to-receiver isolation as high as 195 dB, the passive source producing interference components of the above noted magnitudes pose a serious threat to system performance.
If every cable and connector and every metal-to-metal interface is assumed to be a potential nonlinear interference source, the total number of possible interference generators in any practical situation is very large. As the number of transmitter and receivers, and their respective power levels and sensitivities increase, the system interference problems which are associated with these passive sources, becomes increasingly severe. Obviously, every one of the possible generators does not contribute equally. A bad metal-to-metal interface in a low RF current region is not likely to be a serious source of interference nor are high RF currents themselves, detrimental, unless bad joints in the currents path also exist. The present invention provides an apparatus for identifying, locating, and measuring the various interference sources that are generation as a result of metal-to-metal junctions which are exposed to multiple high amplitude R.F. signals.